Enzymes, which are proteinaceous in nature and are commonly water soluble behave as biocatalysts, regulating many of the chemical reactions which occur in living organisms. Enzymes may also be isolated and used in analytical, medical and industrial applications. For example, they are used in the preparation of food such as cheese and bread as well as in the preparation of alcoholic beverages.
Since enzymes are commonly water soluble as well as being generally unstable, they are subject to deactivation and are difficult to remove for reuse from solutions in which they are utilized. These difficulties lead to an increased cost in the use of enzymes in commercial scale operations due to the necessity for their frequent replacement. In order to reduce the high cost of enzyme replacement, various methods for immobilization (sometimes referred to as insolubilization) of enzymes prior to their use have been devised. This immobilization of the enzyme permits its reuse whereas it might otherwise undergo deactivation or be lost in the reaction medium in which it is used. These immobilized enzyme systems may be employed in various reactor systems, such as in packed columns and stirred tank reactors, depending on the nature of the substrate which is being biocatalytically reacted.
Several general methods as well as many modifications thereof have been described by which the immobilization of enzymes can be effected. For example, materials useful for immobilization of enzymes or cells containing enzymes are disclosed in U.K. Pat. No. 1,444,539. Preferably, the enzymes or cells are treated with a water miscible solvent, such as acetone, dried and then treated with polyethylenimine and glutaraldehyde to make shaped bodies of a water insoluble structure.
More particularly, in U.S. Pat. No. 3,796,634 there is disclosed an immobilization method which involves absorbing a polyamine onto the surface of colloidal sized particles. The polyamine is cross-linked with a conventional amine-reactive cross-linking agent, e.g. glutaraldehyde, and the resulting reaction product is treated with NaBH.sub.4 to reduce the aldehyde groups and thereby prevent any covalent bonding between the aldehyde groups and the enzyme's amino group. Next, the enzyme is absorbed onto the treated surface of the particle at a pH such that the colloidal absorbant bears a net electric charge opposite that of the enzyme molecules so that ionic bonding aids other non-covalent bonding forces. This patent describes the absorbant particles as ranging in size from about 50 to about 20,000 angstroms, preferably from about 100 to 200 angstroms in diameter, with the absorbant material being activated charcoal, hydroxyapatite, alumina C gamma, and bentonite. This system depends on charge interactions for binding the enzyme to the treated particles. This type of bonding is less desirable than the formation of covalent linkages because ionic interactions are susceptible to the environmental conditions relative to this type of linkage such as pH, ionic strength and temperature.
Liu, et al disclose an immobilization method for lactase on granular carbon in Biotechnol. Bioeng. 17, 1695-1696, 1975 which involves absorbing p-aminophenol or 1-phenol-2-amino-4-sulfonic acid to the carbon. These absorbed compounds provide the amino groups with which glutaraldehyde reacts and in turn binds the enzyme. The amino group containing compounds mentioned are monomers which possess chemical and physical properties different from those of a polyamine such as polyethylenimine.
Another group of workers (Cho, et al, Immobilization of Enzymes on Activated Carbon: Properties of Immobilized Glucoamylase, Glucose Oxidase and Gluconolactonase, Biotechnol. Bioeng. 20, 1651-1665, 1978) have also immobilized enzymes on granular carbon by covalent attachment. In this process carbon is activated by a carbodiimide which can be displaced by an enzyme to form an enzyme-carbon complex.
U.S. Pat. No. 4,141,857 discloses a method for enzyme immobilization which involves treating an inorganic porous support material such as gamma-alumina having pore diameters of from about 100 to about 55,000 angstroms and a surface area of about 100 to 500 m.sup.2 per gram with a solution of a water soluble polyamine and contacting the treated support material with a solution of a bifunctional monomeric material, e.g. glutaraldehyde. This treatment leaves the treated support material suitable for reaction with the enzyme so as to form covalent bonds between the enzyme and the pendant aldehyde groups. In Example II of this patent there is described the preparation of an immobilized enzyme conjugate by treating porous alumina spheres sequentially with solutions of polyethylenimine, glutaraldehyde and glucoamylase.
In U.S. Pat. No. 4,438,196 there is disclosed a process whereby enzymes are immobilized on activated granular carbon. The process involves treating the carbon with a polyamine compound having pendant amino groups to cause the polyamine to adhere to the carbon leaving pendant amine groups free to react further. The free amine groups are derivatized by treatment with a difunctional compound having amine reactive moieties, so that free amine groups of the enzyme can be covalently bound to the polyamine via the amine reactive compound.